The present invention relates to an objective lens used for an optical recording/reproducing device that is capable of recording/reproducing large amounts of optical information to/from hologram recording medium.
Recently, such optical recording/reproducing devices using hologram recording medium are in practical use. Features of the optical recording/reproducing device using hologram recording medium are: the device can employ an optical pick-up widely used for optical disc apparatuses which use optical discs such as a CD, a CD-R and a DVD; the device can employ a servo mechanism widely used for the optical disc apparatuses; and the device uses the hologram recording medium. Hereafter, a hologram recording medium having a form of a disc or a card is referred to as a hologram disc.
The objective lens mounted on the optical pick-up provided in the optical recording/reproducing device using the hologram disc is required to be sufficiently corrected for wavefront aberration for a beam from a light source and for a beam reflected from the hologram disc.
A fourier transform lens is known as a lens which is sufficiently corrected for wavefront aberration for a beam entering from a incident surface thereof (a light source side surface) and for a beam entering from an exit surface thereof (a recording media side surface).
Japanese Provisional Publication No. HEI11-133297 discloses a fourier transform lens having a plurality of lenses. Since the fourier transform lens disclosed in the above mentioned publication has the plurality of lenses, if the fourier transform lens is attached to the optical pick-up employed by the optical disc apparatus, the following problems arise.
Firstly, since the fourier transform lens has the plurality of lenses, position adjustments of the plurality of lenses are required. This is troublesome for a worker. Secondly, additional design of a lens frame for holding of the fourier transform lens is required because the fourier transform lens can not be attached to a lens frame mounted on an actuator of the optical pick-up employed by the optical disc apparatus. Use of a new lens frame suitable for the fourier transform lens having the plurality of lenses increases a size of the optical recording/reproducing device.
Accordingly, the objective lens for the optical recording/reproducing device for the hologram disc is required to be a single lens as in the case of the optical disc apparatus.